RFR: 8342578: GHA: RISC-V: Bootstrap using Debian snapshot is still failing

[Fei Yang]

In JDK-8339548, we switched to use Debian snapshot (https://snapshot.debian.org/archive/debian/20240228T034848Z/) for bootstrap. The reason is that we don't have a stable Debian release for RISC-V yet. And Debian "sid" (https://httpredir.debian.org/debian) that we use for debootstrapping RISC-V breaks at that time. This works as expected for about one month. But bad news is that GHA linux-cross-build job for RISC-V starts to fail again this week. Sigh! I guess there might be some change on the distro running on GHA test machines as same debootstrap command still works on my Ubuntu 22.04.

Good news is that that Debian "sid" can now bootstrap for RISC-V. So this simply switches back to Debian "sid". As the version
of dpkg command on GHA machines is old (1.21.1), we need to add one extra option `--no-merged-usr` for bootstrap command to work. This is not necessary for newer dpkg versions like 1.22.6.

Testing:
- [ ] GHA test

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Commit messages:
 - Fix

Changes: https://git.openjdk.org/jdk/pull/21575/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21575&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8342578
  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/21575.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21575/head:pull/21575

PR: https://git.openjdk.org/jdk/pull/21575